flat roof insulation and condensation

Hello I'm a novice diyer and need some advice regarding replacing the plaster board ceiling in my extension. The property has a 2 story extension (40 years old) which is 1.8mx4m and has a felt flat roof which I can't get on top of to inspect. There appears to be a condensation problem all around the joint of ceiling to wall. I have poked a hole to check above the old plaster board and there is no insulation, which explains why its freezing in there. It appears to be a cold deck construction but the plasterboard has a moisture barrier but from the outside I cannot see if there has any cross ventilation in the construction.

My questions are what is the best way to go about insulating the ceiling and reducing the condensation recurring. Should a vcl be used and if so what material and how should it be installed. I don't have the budget to get someone in so will be doing this work myself.